Process for the separation of nitrogen and oxygen from air by fractional distillation



Sept. 5, A1967 M. F. M. R. sTRElcH ETAL 3,339,370

PROCESS FOR THE SEPARATION OF NITROGEN AND OXYGEN FROM AIR BY FRACTIONAL DISTILLATION Filed June l5, 1964 2 Sheets-Sheet 1 ATTORNEY M. F. M. R. sTRElcH ETAL 3,339,370 PROCESS FOR THE SEPARATION OF OXYGEN F Filed June l5, 1964 Sept. 5, 1967 NITROGEN AND ROM AIR BY FRACTIONAL DISTILLATION 2 Sheets-Sheet 2 @www AMMIIMVLI I I Nm mNII m 1. oww ovm Ir A E MNB GNN

INVENTORS Morfn FM.R.`Streich Ralph C. Tuon ATTORNEY United States Patent iice 3 339,370 PROCESS FOR THE SEPARATION F NITROGEN AND OXYGEN FROM AIR BY FRACTIONAL DIS- TILLATION Martin F. M. R. Streich, Frankfurt am Main, Germany,

and Ralph Charles Tutton, Teddington, England, assignors to Conch International Methane Limited, Nassau, Bahamas, a company of the Bahamas Filed June 15, 1964, Ser. No. 375,979 Claims priority, application Great Britain, Nov. 12, 1963, 44,604/ 63 4 Claims. (Cl. 62-14) This invention provides -a process for the sepa-ration of nitrogen and oxygen from air by fractional distillatlon, first at a higher pressure and then at a lower pressure, in which gaseous nitrogen separated in a lower .pressure fractionation column is used to cool air flowing to a higher pressure fractionation column and gaseous nitrogen separated in the higher pressure fractionation column is compressed at a temperature below minus 50 C. and cooled -by indirect heat exchange with an external refrigerant and is then expanded 4back into the higher pressure fractionation column.

This process is particularly applicable when a large volume of a cold liquid, such as liquefied methane or liquefied oxygen, is available and requires gasification. By using such a cold liquid as the external refrigerant in the present process, considerable savings in the power required to separate nitrogen and oxygen from air are achieved as compa-red with conventional air separation processes, and at the same time the available liquefied gas is regasified for use, for example in the case of liquelied methane for use as towns gas or for towns gas enrichment or for reforming to towns gas.

Moreover, further power is saved by the novel step of compressing the ygaseous nitrogen from the higher pressure fractionation column at low temperatures, i.e. below minus 50 C. Preferably this compression takes place at a temperature of about minus 150 C. In this specification when we refer to 'compressing at or below a certain temperature we are referring to the temperature a-t the inlet to the compressor, i.e. the suction temperature.

The gaseous nitrogen separated in the higher pressure fractionation column may be cooled by an external refrigerant in any one of a number of ways. The cooling by the external refrigerant may take place before or after the compression of this gaseous nitrogen stream or both before and after such compression. When the compression is carried out in stages, the said cooling may also take place between the stages.

Where the external refrigerant is a hydrocarbon such as liquefied methane, it may be preferable to employ an inert gas such as nitrogen, argon or helium as a heat transfer medium between the hydro-carbon and the circulating nitrogen stream from the higher pressure fractionation column. Such heat transfer medium may itself operate in a closed gas expansion refrigeration cycle. The use of such an inert gas as heat transfer medium .between the ultimate external refrigerant and the circulating nitrogen stream minimizes the possibility of hydrocarbon leakage into the fractionation tower should there be any leakage in any of the heat exchangers utilizing the hydrocarbon. Such -a system is simply a more complicated conception of the invention as defined above, since the inert gas operating in a closed gas expansion refrigeration cycle can itself be looked upon as the external refrigerant cooling the nitrogenflowing from the higher pressure fractionation column.

Thespeciiicpnature of the invention, as well as other objects and advantages thereof, will clearly appear from Patented Sept. 5, 1967 2 a description of a preferred embodiment, as shown in the accompanying drawings, in which:

FIG. 1 is a ilow diagram illustrating the principle of the invention; and

FIG. 2 shows an alternative form of the invention.

Referring to FIG. 1, air enters the plant via conduit 1 and -blower 2 and then passes through dust filter 3 and heat exchangers 4 and 5. In heat exchanger 4 the air is cooled by indirect heat exchange with cold water and in heat exchanger 5 it is cooled by indirect heat exchange with cold natural gas flowing through conduit 6 as will 'be described in more detail below. Any water condensing out of the -air in heat exchangers 4 and 5 is removed via valves 7 and `8 respectively. Heat exchanger 5 may be a reversing heat exchanger or one of a pair of regenerating heat exchangers so that freezing of air humidity can be dealtwith.

The air leaving heat exchanger 5 is at a temperature of about minus C. and then passes through a silica .gel bed 9 to remove any remaining humidity. It then passes through heat exchanger 10 in which it is cooled to minus C. by heat exchange with cold natural Agas flowing through conduit 11 as will be described in more detail below.

The air at minus 140 C. is then compressed to about 5.5 atmospheres in ocmpressor 12 and fed through heat exchanger 13 in which it is cooled to minus 172 C. by indirect heat exchange with waste gaseous nitrogen flowing through conduit 14 and circulating gaseous nitrogen flowing through conduit 15. Heat exchanger 13 may be a reversing exchanger in one of a pair of regenerating heat exchangers so that carbon dioxide freezing out at this stage will be removed; however, heat exchanger 13 may alternatively be a nonreversing heat exchanger and in this case the carbon dioxide would be removed either by low temperature adsorption or by caustic scrubbing after blower 2 or by any other conventional carbon dioxide removal system.

The air at minus 172 C. then passes through a hydrocarbon absorber 16 to remove any hydrocarbon contaminants, and then passes into higher pressure fractionation column 17 operating at about 5.5 atmospheres. Above column 17 is a lower pressure column 29. From the top of column 17 a stream of gaseous nitrogen at minus 178 C. is taken off through conduit 1'8, part of which stream flows Via conduit .15 through heat exchanger 13 to cool the air stream as described above, and the other part flows via conduit 19 through heat exchanger 20 in which it cools compressed gaseous nitrogen as will be described below. The two parts of the said stream rejoin one another in conduit 21 at a temperature of about minus 73 C. and are cooled in heat exchanger 22 to minus 153 C. by heat exchange with natural gas as will be described below. The stream of nitrogen at minus 153 C. is compressed to 19 atmospheres in compressor 23, cooled again in heat exchanger 24 against natural gas, as will be described below, compressed to 71 `atmospheres in compressor 25 and cooled again partly in heat exchanger 20 against the cold nitrogen gas in conduit 19 and partly in heat exchanger 26 against natural gas, as will be described below. The compressed nitrogen stream then has a temperature of about minus 158 C. in conduit 27, which feeds it back to the higher pressure fr-actionation column 17 through expansion valve 2'8. The expansion through valve 28 causes a large drop in temperature providing refrigeration for the operation of column 17.

In column17 a liquid containing about 40% oxygen collects at the bottom. The gases arising in this column are partially condensed to form a reliux, the distillation plates in the column being omitted from the drawing in the interests of simplicity. The uncondeused nitrogen at the top of the column is taken off via conduit 18 as already described, lbut some nitrogen is condensed in the reflux condenser 30, which also acts as the boiler for the lower perssure fractionation column 29. This condensed nitrogen collects in the annulus 31 from which it is fed via conduit 32 and expansion valve 33 to column 29. Part of the condensed nitrogen may be withdrawn at 31a from annulus 31 as product if required. The liquid collected at the bottom of column 17 is fed to column 29 via conduit 34 and expansion valve 35, after passing through a silica gel filter 34a to remove traces of hydrocarbons and carbon dioxide.

In column 29, which operates at about 1.4 atmospheres, liquid oxygen collects at the bottom and is drawn off as product via conduit 36, while gaseous nitrogen is taken off overhead via conduit 14 and used to cool the incoming air in heat exchanger 13.

Reverting to the gaseous nitrogen stream which is taken olf from the top of column 17 via conduit 18, this is, as already explained, cooled by an external refrigerant, in this case cold natural gas. Liqueiied natural gas at its boiling point at atmospheric pressure is fed into the system via conduit 37 and pumped up to 70 atmospheres. pressure in pump 38, when its temperature will be about minus 160 C. It is then divided into two streams in conduits 39 and 40. In conduit 39 it flows through heat exchanger 26, in which it cools the nitrogen stream circulating in line 27a from and back to column 17, and then is fed via conduit 6a into conduit 6 in which it precools the air feed in heat exchanger 5. From conduit 40 the natural gas is divided into three streams in conduits `41, 42 and 43, the first two of which are used to cool the circulating nitrogen stream in heat exchangers 22 and 24 and then are fed back via conduit 6b into conduit 6 downstream of heat exchanger 5, and the third of which is used to cool air in heat exchanger 10 and is then fed back via conduit 6c into conduit `6 upstream of heat exchanger 5. The natural gas exits from the plant via conduit 6 at about 70 atmospheres pressure, and is available vfor pipeline distribution to its point of use, if necessary, after warming to ambient temperature.

A plant, as in the accompanying drawing, producing 500 metric tons of liquid oxygen per day would have a power requirement of 4,630 kilowatts. In addition, 102 million standard cubic feet per day of liquefied natural gas would be gasifed. A conventional plant producing 500 metric tons `of liquid oxygen per day would have a power requirement of about 15,000 kilowatts, demonstrating the considerable savings afforded by the present invention.

It will be appreciated that the process described above with reference to the accompanying drawing can readily be adapted for the production of gaseous oxygen instead -of liquefied oxygen. For example, if gaseous oxygen at about 16 atmospheres pressure is required as product, the liquefied oxygen, leaving through conduit 36, can be pumped up to, say, 19 atmospheres pressure, used to assist in cooling the circulating nitrogen stream by passing the pressurized liquid oxygen through heat exchanger 20 in which it would `be vaporized and converted to gaseous oxygenat the required pressure. In such a case, of course, less liquefied natural gas would be required to operate the -process as compared with the case when liquid oxygen is being produced.

Alternatively, gaseous oxygen may be withdrawn from the lower part of column 29 and then pass through heat exchanger 13 or 20 and then compressed to about 16 atmospheres with or Without inter-cooling. The cooling medium could be cold nitrogen in one of heat exchangers 22, 24 or 26.

It will -be clear that the heat exchange arrangements between the circulating nitrogen stream and the natural gas described with reference to the accompanying drawing are capable of many variations and the particular arrangement adopted will depend upon the precise operating conditions. For example, the streams of nitrogen in conduits 15 and 19 can be reunited downstream of heat exchanger 22 so that only the stream in conduit 15 passes through heat exchanger 22 and the stream in conduit 19 by-passes that heat exchanger.

FIG. 2 illustrates an alternative manner of carrying out the process of the present invention, in which the external refrigerant is a hydrocarbon and an inert gas, such as nitrogen, is used as heat exchange medium between the hydrocarbon and the nitrogen stream from the higher pressure fractionation column. The additional nitrogen line is depicted in FIG. 2 by -a heavier line than the rest of the flow circuit merely in order to show more clearly the portion which has been changed. In FIG. 2, the flow of air and of oxygen and nitrogen from the fractionation columns is the safme as in FIG. 1, but heat exchangers 10, 22, 24 and 26 are replaced by evaporative heat exchangers, in which the air and the nitrogen from the high pressure fractionation column respectively are cooled against evaporating liquid nitrogen operating in a vapor compression refrigeration cycle as follows:

Liquid nitrogen in conduit 50 is divided into four streams in conduits 51, 52, 53 and 54. In conduit 51 it flows through expansion valve 55 and then through evaporative heat exchanger 26a, in which it cools the nitrogen stream circulating from and back to column 17 and then is fed into heat exchanger 5. The liquid nitrogen in conduits 52 and 53 pass through expansion valves 56 and 57 respectively and hence through evaporative heat exchangers 24a and 22a respectively, in which it cools the circulating nitrogen stream from column 17 and from which it is then fed back into conduit `6 downstream of heat exchanger 5. The liquid nitrogen in conduit 54 passes through expansion valve 58 and thence through evaporative heat exchanger 10a, in which it cools air and is then fed back into conduit 6 upstream of heat exchanger 5. All these streams of nitrogen, which are now in the gaseous phase, join together in conduit 59, are compressed in compressor 60 and liquefied in heat exchanger 61 against liquefied natural gas passing through conduit 62 from a pump 63, and the liquid nitrogen iiows into conduit 50y to complete thecycle.

The present invention can be applied to a three-stage fractionation column `wherein the nitrogen is withdrawn from the Ihighest pressure column. In this case, one could use a heat exchange system similar to that described with reference to the accompanying drawings, but wherein the nitrogen, 'before compression, is under a pressure of about 10 atmospheres. In such a cycle it is also possible to compress only part of the air to 6 atmospheres and the other part to a higher pressure, for example, 10 atmospheres, using the nitrogen from the highest pressure column for recycling.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of our invention as defined in the appended claims.

What we claim is:

1. A process for the separation of nitrogen and oxygen from air by fractional distillation, first at a higher pressure and then at a lower pressure,

(a) in which the gaseous nitrogen is separated from air in a lower pressure fractionation column and is used to cool air flowing to a higher pressure fractionation column and (b) gaseous nitrogen separated in the higher pressure fractionation column is compressed at a temperature below minus 50 C.

(c) and is cooled by indirect heat exchange with a liquefied natural gas refrigerant from a source external to the separation system (d) and is then expanded back into the higher pressure fractionation column,

(e) wherein the gaseous nitrogen separated in the higher pressure fractionation column is divided into two streams prior to said compression, and is heat exchanged in one of said streams with incoming air feed for the system, and is heat-exchanged in the other of said two streams with compressed and cooled gaseous nitrogen from step (d), after which the two streams are reunited prior to said compression, said compression being in at least two stages with a heat exchange prior to and intermediate the stages, the external natural gas refrigerant being in parallel low in the latter heat exchange.

2. A process for the separation of nitrogen and oxygen from air by fractional distillation, rst at a higher pressure and then at a lower pressure,

(a) in which the gaseous nitrogen is separated from air in a lower pressure fractionation column and is used to cool air flowing to a higher pressure Afraction-ation column and (b) gaseous nitrogen `separated in the higher pressure fractionation column is compressed at a temperature below Iminus 50 C.

(c) and is cooled yby indirect heat exchange with a liquefied methane refrigerant from a source external to the separation system (d) and is then expanded back into the higher pressure fractionation column,

(e) wherein the gaseous nitrogen separated in the higher fractionation col-umn is divided into two streams 'prior to Isaid compression, and is heatexchanged in one of said streams with incoming air feed for the system, and is heat-exchanged in the other of said two streams with compressed and cooled gaseous nitrogen from step (d), after which the two streams are reunited prior to said compression, said com-pression being in at least two stages with a lheat exchange prior to and intermediate the stages, the external methane refrigerant being in parallel ow in the latter heat exchange.

3. A process as claimed in claim '2 in which the gaseous nitrogen separated in the higher pressure fractionation column is compressed at a temperature of about 150 C.

l4. A process as claimed in claim 2 wherein the liqueed methane refrigerant is vaporiz/ed to gaseous form in the process and is discharged from the system as an additional product.

References Cited UNITED STATES PATENTS 2,685,181 8/1954 Schlitt 62-40 X 3,058,314 10/ 1962 Gardner u 62-40 X 3,183,677 5/1965 ATafreshi 62-40 X 3,203,191 8/1965 French 62-40 X 3,216,206 11/ 1965 Kessler 62-13 FOREIGN PATENTS 968,603 6/ 1948 France.

NORMAN YUDKOFF, Primary Examiner. V. W. PRETKA, Assistant Examiner. 

1. A PROCESS FOR THE SEPARATION OF NITROGEN AND OXYGEN FROM AIR BY FRACTIONAL DISTILLATION, FIRST AT A HIGHER PRESSURE AND THEN AT A LOWER PRESSURE, (A) IN WHICH THE GASEOUS NITROGEN IS SEPARATED FROM AIR IN A LOWER PRESSURE FRACTIONATION COLUMN AND IS USED TO COOL AIRE FLOWING TO A HIGHER PRESSURE FRACTIONATION COLUMN AND (B) GASEOUS NITROGEN SEPARATED IN THE HIGHER PRESSURE FRACTIONATION COLUMN IS COMPRESSED AT A TEMPERATURE BELOW MINUS 50*C. (C) AND IS COOLED BY INDIRECT HEAT EXCHANGE WITH A LIQUEFIED NATURAL GAS REFRIGERANT FROM A SOURCE EXTERNAL TO THE SEPARATION SYSTEM (D) AND IS THEN EXPANDED BACK INTO THE HIGHER PRESSURE FRACTIONATION COLUMN, 